Coal is a solid fuel of heterogeneous composition. It is constructed as a mixture of pure hydrocarbons, oxygenated hydrocarbons, complex hydrocarbons with other elements attached, inorganic minerals, and water. Other solid fuels, such as municipal waste, are also usually largely composed of mixtures of complex hydrocarbons, inorganic materials, and water. In some cases, heavy liquid fuels, such as number six fuel oil, and similar fuels, also can be compared to coal chemically and in their combustion characteristics. The present invention includes all such fuels which produce appreciable flyash in their combustion gas steams. Subsequent discussion concentrates largely on a description of coal, but should also be interpreted to include the combustion of any fuel which produces a flyash laden gas stream and which it is desired to remove volatile noxious species, particularly mercury.
The combustion of coal and other similar fuels as currently practiced produces by-products: a combustion gas often primarily composed of nitrogen, carbon dioxide, water vapor, oxygen, and smaller amounts of vaporized inorganic elements and compounds, and inorganic products of combustion (ash). Often the combustion gas carries ash along with it. Combustion gas borne ash is generally known as flyash.
Flyash is usually composed of common oxygenated inorganic compounds made of silica, aluminum, calcium, magnesium, iron, sodium, potassium, titanium, and sulfur. Any of the naturally occurring elements may be found in flyash, but those listed above usually account for more than 90% by mass of commonly occurring flyash, and aluminosilicate and calcium compounds often account for more than 80% of the mass. There are often trace elements contained in the flyash. The trace elements are fundamental constituents of the coal or other fuel, usually not exceeding about 100 parts per million in concentration, and can be nearly any of the remaining natural elements. One of the more common trace elements found in coal and the combustion byproducts of some solid fuels, such as some municpal waste fuels, is mercury.
Three other common properties of flyash found immediately upon its production in a combustion process are: relatively small average particle size (.about.0.5 .mu.m to .about.100 .mu.m), relatively large surface area per particle, and a largely anhydrous condition, all due to the rigor of combustion.
The small average particle size, and large particle surface area yield a very large surface area per unit mass of flyash. As long as the flyash can be maintained in a largely anhydrous condition, the flyash often acts as a sorbent for extremely small particulates, fumes, or vapor phase species which can be sublimated, condensed, or chemisorbed onto or into the flyash particles.
The ability of largely anhydrous flyash to absorb gases or fumes such as vapor phase mercury is not dependent upon the flyash's location in respect to the gas stream containing the mercury. That is, flyash which is being borne by the combustion gas stream can be expected to absorb mercury at rates similar to flyash which is concentrated on the surface of a filter bag as a filter cake. Flyash, which is part of such a filter cake, is very loosely bonded to other flyash particles, and forms a very porous mass. The exposed and available for reaction (activity) surface area of individual flyash particles in such of a filter cake approximates the same as an individual flyash particle which is airborne. The mass to be captured, such as mercury, is often either in a vapor state, where it is at a molecular size, or of a vapor condensed fume, of an often micron, or sub-micron size. Such masses are in gaseous kinetic or Brownian motion, respectively, and "see" very large gaps in even a large accumulation of supermicron size objects as occurs with a flyash filter cake. The ability to trap such material as mercury is not due to the accumulation of flyash acting as a sieve, but due to the high surface area of the flyash to act as sites of condensation, sublimation, or chemisorption.
Although a preferred embodiment of the present invention is the use of flyash which has been modified by capture in a particulate separation device, such as a cyclonic separator, etc., it should also be understood that a reaction agent which mimics the properties of the modified flyash, i.e. particles ranging in size from about 1.0 .mu.m to about 100 .mu.m, having a large surface area per particle due to porosity, and are often largely composed of aluminosilcate mineral phases, can also be used to capture mercury in a combustion gas stream by the present invention. The capture of flyash in a particulate separation device modifies the raw or natural flyash stream by coarsening its particle size distribution. Particles which are less than 1.0 .mu.m are ususally not captured by the particulate capture device.
There has been a growing concern that some trace elements often found in coal, such as mercury, can cause degradation to the natural environment if exhausted unabated during coal combustion to the natural environment. There is current art in capturing mercury and other trace elements during coal combustion, often using activated carbon as the sorbent, but the current art is often too expensive for economic usage.
There is evidence in published literature that flyash can be used as an effective sorbent of some of these trace elements, particularly mercury. First cited is U.S. Pat. No. 4,273,747, Rasmussen, which explains two examples of using flyash to remove mercury from a combustion gas stream. In Rasmussen 747, largely anhydrous flyash incidentally removes mercury from a gas stream through non-deliberate contact, the flyash first contacting the mercury which is borne in a combustion gas stream, and second while the flyash is trapped as a filter cake on the surface of a filter bag. (Rasmussen 747 teaches reducing the temperature of the combustion gas stream to activate the sorption of the mercury vapor onto flyash, and does not discover means to increase the mercury sorption in the "active" zone.) In both cases, the flyash remained largely anhydrous and was described as a dry free flowing powder. Rasmussen 747 reported a nominal 90% reduction in mercury content compared to the base combustion gas. The combustion gas was generated by burning municipal incinerator waste. The combustion gas stream had a relatively high mercury concentration of 100 .mu.g/Nm.sup.3.
Second cited is U.S. Department of Energy report, Comprehensive Assessment of Toxic Emissions from Coal-Fired Power Plants, contract No. DE-AC22-93PC93255, 1993. This report indicates that a nominal 60% reduction in mercury content was experienced during normal operating conditions across a baghouse used in industrial application to collect coal produced flyash. In this case, the mercury concentration of the combustion gas stream was a relatively low 6.4 .mu.g/Nm.sup.3 (as reported on page ES27). This report states on page ES-24:
"A review of the average inlet and outlet metals emission rates measured using EPA Draft Method 29 indicates greater than 98% removal of all metals through the baghouse with the exceptions of cadmium, selenium, and mercury. On the average, 60% of the total mercury in the flue gas stream was removed by the baghouse. The measured mercury removal determined by EPA Draft Method 29 is supported by the quantity of mercury determined in the corresponding baghouse hopper ash samples obtained for each run. The total baghouse mercury mass emission rate when added to the total mercury mass rate determined for the corresponding bag house ash stream, compared within 2,8, and 23 percent of the total mercury measured in the baghouse inlet flue gas stream using EPA Draft Method 29 for Runs 1,2, and 3, respectively."
Contemporary chemical science teaches that chemical reaction rates are often dependent upon reactant concentrations, with reaction rates initially higher with higher concentrations, then decreasing exponentially with decreasing reactant concentrations. Thus, the different removal efficiencies of the two examples cited above are to be expected based on conventional knowledge. In fact, the manipulation of concentrations and contact times of the flyash and mercury as reactants in novel methods is a central feature of the current invention.
A baghouse is a commercially available device used to separate particulate material from gas streams at industrial scales of operation. Baghouses use a collection of often fabric filters, similar to common household vacuum cleaners, but at a much larger scale, to strip gas borne particulate matter onto a filter surface, allowing the largely particulate-free gas to continue through the filter surface.
During operation of the baghouse particulate matter builds up on the surface of the filter(s). This build-up is commonly known as the bag's "cake." Cakes are frequently allowed to build up to thicknesses of approximately 0.25 inch or somewhat more between intervals of cleaning.
Bags in operational baghouses are cleaned of cake buildup at periodic intervals: determined by variables of operation and engineering design. The cleaning process often involves blowing air backwards through the bag filters, or shaking bags, or banging the tops of the bags, all of which cause a substantial portion of the filter cake to drop off of the bags. This causes a periodic subsidence in the thickness of the filter cake.
That a reduction of 60% in mercury content can be achieved with a contact time as short as experienced in the gas/filter bag interface (estimated to be roughly four milliseconds: assuming a superficial velocity of five feet per second, and a filter cake of 0.25 inch yields a contact time of: T=(0.25/12)/5=0.004 seconds.), is indicative of the effectiveness of largely anhydrous flyash as a sorbent of mercury and other trace elements.
Chemical reactions involving the interaction of a dilute reactant often utilize the concept of a "CT" value, where C is a relative concentration value, and T is a measure of time. The product of C times T yields a measure of the relative reaction, based upon the normal linearity of reaction rate constants found in reactions where at least one reactant is dilute.
The concentration of mercury can be considered relatively constant for a given coal source and under normal combustion conditions. In order to decrease the amount of mercury released to the environment in the combustion gas stream, increases are needed in the contact time T, and the concentration of the other reactant, flyash, expressed as C, i.e., increases in the CT value would be expected to increase the amount of mercury sorbed onto/into the flyash.
Thus, what is needed is a process and/or apparatus which allows significant increases in the CT value of a practical and economic nature, particularly using a largely anhydrous flyash as the sorbent.
Current art in mercury sorption often uses activated carbon (or coke) as the sorbing reactant. There are at least three activated carbon/coke based technologies in current art which remove air toxics. They are the Joy/Niro spray dryer adsorption system, the System Dusseldorf, and the GE-Mitsui-BF dry Deso.sub.x /DeNO.sub.x /Air Toxics removal process. In most of these applications, the mercury-containing combustion gas is forced through a deep bed of activated coke: activated coke is just an "industrial" version of activated carbon. True activated carbon is expensive stuff which finds application in every thing from pharmaceuticals to potable water systems. Activated coke is still quite expensive (about 20% the cost of activated carbon), its current market price is about 1,000 per ton. Activated carbon and activated coke both have some physical properties similar to flyash, primarily a large surface area per unit mass. The large surface area is due to a micropore structure present in activated carbon, which acts as a sublimation or chemisorption surface and/or molecular sieve. The ability for flyash to similarly act as an effective sorbent is likely due to these physical similarities.
The use of activated carbon/coke, however, suffers from significant drawbacks:
1. Activated carbon/coke is expensive. The current market for activated carbon at an industrial scale is between 1,000 to 5,000 per ton. This results from an expensive attritive process used in the manufacture of the activated carbon/coke, as well as the cost of the process feed stock, which is often an expensive coking coal. PA1 2. Substantial pressure drop reduces plant efficiency. In order to obtain effective mercury reductions (which equates to adequate CT values), the relatively large depth of the bed of activated carbon/coke often results in substantial pressure drop of the combustion gas through the bed, which necessitates increased fan power, which reduces the net efficiency of the coal combustion system. PA1 3. Mechanical weakness of activated coal/coke and concomitant problems. Many of the activated carbon/coke processes use "lump" sizes (excluding Moller, etal, 698). Lump sized activated carbon/coke uses an average particle size approximately one inch (2.5 cm). Because activated carbon/coke sorbs mercury from the combustion gas stream, it must be periodically replaced. Activated carbon/coke are fragile solids, and during the mechanical handling associated with replacement are subject to breakage. This problem is inconsequential with sorbent flyash, which remains as readily fluidized, small particulate matter. PA1 4. Inefficient utilization of reactant. Because of the high cost of activated carbon/coke, the entire process must be designed to minimize the loss of sorbent, thus abrogating the possibility of using very fine (and thus ultra-high surface area) particulate. This increases the required mass of sorbent: and cost.
The use of a flyash based sorbent system can be designed to have none of the above limitations. Since flyash is generated on site as a fundamental property of coal combustion (and flyash is also generated by the combustion of other fuels with significant inorganic content), its availability is nearly infinitely more economic than activated carbon/coke. This fact allows the development of a mercury sorption system which is simple, practical, economic, and innovative.
Different types of fuels exhibit different levels of mercury concentrations. Some municipal and medical waste incineration systems are known to generate mercury concentrations in the combustion gas stream of about 100 .mu.g/Nm.sup.3. This places different constraints on the type of mercury sorption system which can be used. Combustion of coal often contains a small fraction of mercury present in many municipal waste combustion streams, (in many cases about 5% of that found in some municipal waste combustion streams). As a result, and partly because mercury concentration in coal combustion systems have been at the limit of detectibility, more expensive systems (i.e. activated carbon/coke) for mercury sorption have found practical application in municipal waste incinerators, but are only in limited use for larger industrial scale coal combustion systems. What is needed is a practical, economic means of removing mercury from low concentration (i.e. industrial coal) combustion gas streams. When such a method becomes available, it can be extended even more economically to high mercury (i.e. municipal waste) concentration systems.
The current invention is a practical, economic means of removing mercury from low mercury concentration combustion gas streams. An improvement in the art of mercury sorption in coal based combustion gases is described in the summary of the invention.
U.S. Pat. No. 5,270,015 Rochelle, etal., teaches a method for the removal of sulfur and acid components from a flue gas stream. Alkali components which can include flyash are extracted from the gas stream by an ash collection device and mixed with water to create a slurry composed of water and alkali solids. The use of water to create a slurry is an essential part of Rochelle, 015. The present invention does not use water in the process. The present invention is limited to largely anhydrous flyash or similar particulate matter, in order to have its activity as high as possible. Activity in this case is related to reaction surface area for a gaseous/solid reaction. Any appreciable moisture content reduces available surface area for mercury removal.
U.S. Pat. No. 4,863,489, Suggitt teaches a method for the removal of mercury from a synthesis gas stream using a pressurized solvent scrubber: mercury is condensed and washed from the gas stream. Further, Suggitt, 489 does not use largely anhydrous flyash as a reactant. The current invention does not propose to remove mercury or mercury compounds from a synthesis gas stream, but is limited to combustion gas streams, specifically flyash laden combustion gas streams.
U.S. Pat. No. 4,889,698, Moiler, etal, teaches a method for the removal of mercury and noxious polyorganic matter by the use of a suspended powdery activated carbon in a combustion gas stream. The present invention avoids the use of powdery activated carbon and obtains the satisfactory removal of mercury by discovering methods that engineer the contact of largely anhydrous flyash with the mercury laden combustion gas. The use of flyash is preferred over Moller 698 due to its practicality and economy.
Rasmussen, U.S. Pat. No. 4,273,747, teaches a method for the removal of mercury from a flyash laden combustion gas stream which uses the atomization of water to depress combustion gas temperature. In Rasmussen 747, the mercury is removed by combustion gas suspended flyash, but through incidental contact. The reason Rasmussen 747 depresses the combustion temperature is to place the reactants in a thermodynamic regime which favors increased reaction rates. The present invention could be used in conjunction with Rasmussen 747, without intersection of concept boundaries. Rasmussen 747 teaches the depression of combustion gas temperature, whereas the present invention teaches new methods of engineering the contact of flyash to satisfactorily remove mercury. The concept of using the flyash as a primary discovery was not claimed by Rasmussen 747. The present invention does not claim to protect the discovery of flyash as a mercury sorbent, since this was clearly disclosed by Rasmussen 747 in 1981. But, engineering the controlled removal of mercury by contact with flyash through a novel process, and specific embodiments described in detail, are claims of the current invention.